The present invention is directed to a system of and method for imaging an object using a magnetic field and more particularly to a system of and method for generating a magnetic field with optimized orthogonal gradients for imaging the object. The invention is particularly, but not exclusively, intended for use with Magnetic Resonance Imaging (MRI).
Diffusion-weighted MRI is presently the only available noninvasive method which provides information depicting molecular displacements comparable to cell dimensions. It can provide physiological information unattainable by other conventional imaging modalities. Diffusion-weighted imaging based on water molecular diffusion can rapidly identify and quantify ischemic changes within several minutes following the onset of ischemia. Numerous efforts have been made to produce diffusion-weighted images for clinical use, particularly for the detection of brain ischemia. Because the diffusion tensor trace is invariant upon rotation of the coordinate system and is independent of the tissue's local fiber-tract direction, the diffusion-weighted trace image is an isotropically weighted MRI image in which the intensity is relative to the trace of the diffusion tensor.
Determination of the diffusion trace usually requires separate measurements of the diffusion constants in the three orthogonal gradient directions. More rigorously, the trace can be determined from a full diffusion tensor which requires more than three measurements.
A diffusion tensor which provides information to characterize molecular displacements in three dimensions can be used to interpret anisotropic tissue, such as brain white matter, both in structure and function. The diffusion tensor is therefore introduced as an additional MRI contrast parameter. That is usually accomplished by combining data from several separate measurements with diffusion weighting in different directions. Sensitivity to diffusion effects is proportional to the square of the magnetic field gradient strength. Most clinical whole-body scanners have gradients of only 10-30 mT/m. As a consequence, longer gradient pulses are required, which may prolong the echo time (TE) and result in major signal losses and motion artifact exacerbation.
The diffusion-weighting factor b is a user-controlled parameter which defines the extent of the diffusion weighting. The efficiency of a pulse sequence for diffusion weighting is proportional to the diffusion-weighting factor with the same TE. Clearly, with greater magnetic field gradient strength, a shorter TE will provide for the same diffusion weighting and result in higher SNR diffusion-weighted images. However, the ability to increase the gradient is highly constrained. MRI techniques for the measurements of the diffusion tensor proposed so far are highly susceptible to background noise contamination, and their usefulness has therefore been limited.
To improve the accuracy in determining the diffusion tensor acquired with the fewest diffusion-weighted images, a number of gradient schemes have been employed recently. One such scheme includes some invariant measures of diffusion tensor. Another employs the tensor dot product of the anisotropic part of the diffusion tensor with itself, D:D, as a scale measure of the magnitude of diffusion anisotropy in a voxel, and then obtains information about the pattern of diffusion tensors in an image using tensor algebraic approaches. Unfortunately, most of these anisotropy measurements are time-consuming. For that method to be clinically practicable, one must reduce the time required to retain adequate estimates.
Still another scheme is a tetrahedral sampling scheme to analyze diffusion tensors of samples with axially symmetrical diffusion, but that scheme does not satisfy general anistropical diffusion samples. Yet another applies six diffusion gradient vectors of equal magnitude with uniform oriented distribution in a cuboctahedron scheme for general diffusion samples. However, the latter two schemes use triclinic combining gradients.
To reduce experiment time and simplify post-processing procedures, several gradient pulse sequences with single-shot scans to produce isotropic diffusion-weighted images have been proposed recently. One such sequence uses an orthogonality relationship as one basic condition and reports a number of gradient waveforms for diffusion trace weighting optimized from two-step numerical methods. Another uses an orthogonality relationship to design an original gradient waveform for single-shot isotropic diffusion weighting. Still another designs different single-shot diffusion trace weighted MR gradient waveforms applying a set of combining gradients with peak amplitudes of 3G.sub.0, where G.sub.0 is the peak amplitude of a single physical gradient. A fourth implements their single-shot trace-weighted acquisition scheme for isotropic diffusion echo-planar imaging on a clinical scanner.